Reference for Yevamot 109:12
ודלמא אין לה בנים אסורה לעלמא ושריא ליבם יש לה בנים שריא לעלמא ושריא ליבם אי נמי אין לה בנים מצוה יש לה בנים רשות
while the other refers to a brother's wife who had no children during the lifetime of her husband! — The case of one who had no children during the lifetime of her husband may be deduced from the statement of R. Huna.<span class="x" onmousemove="('comment',' Supra 54b; and no special text is needed for the purpose. ');"><sup>24</sup></span> Might not both<span class="x" onmousemove="('comment',' The two sections of the verse cited. ');"><sup>25</sup></span> still speak of the wife of a paternal brother, the one referring to a brother's wife who had children during the lifetime of her husband and the other to one who had children after the death of her husband! — The case of one who had children after the death of her husband requires no Scriptural text; for since the All Merciful said that she who had no children was permitted, it is obvious that if she had children she is forbidden.